An Al—Fe-based 8000-series alloy has a high strength and a relatively good anodic oxide coatability, so has been used as the material for producing a decorative sheet for building material or a housing of electronic equipment etc. This is formed into a sheet material, which is then cut into the desired dimensions and shaped according to need, then treated at its outer surface to form an anodic oxide coating and used as a building material. Further, after forming the anodic oxide coating, the sheet is given gloss by buff-polishing and also used as the housing of electronic equipment etc.
An aluminum alloy sheet which is based on both the 8000-series alloy and 5000-series alloy to secure strength while is prescribed in the amounts of Si, Fe, Mn, etc. to make the anodic oxide coating color a light gray or an aluminum alloy sheet which is based on 8000-series alloy to secure strength while further having added Mn, Si to make the anodic oxide coating color uniform and a method of production of the same etc. have also been developed. Further, recently, an aluminum alloy sheet based on the 1000-series alloy and improved in uniformity of color of the anodic oxide coating and its method of production etc. have also been developed.
For example, PLT 1 describes an aluminum alloy sheet which is comprised of an aluminum alloy which contains Fe: 0.1 to 1.0 wt %, Si: 0.01 to 0.5 wt %, and Mn: 0.05 to 1.0 wt % and has a balance of Al and unavoidable impurities and which is treated by anodic oxidation to form a uniform color and describes to subject an aluminum alloy ingot of the above composition to a homogenization at 400 to 600° C., hot roll this aluminum alloy ingot to obtain a hot rolled sheet, then cold roll this hot rolled sheet, including process annealing and/or final annealing, and treat it by anodic oxidation to obtain an aluminum alloy sheet which forms a uniform color.
On the other hand, in an aluminum alloy sheet which is based on a 8000-series alloy, while depending also on the homogenization treatment conditions, it is known that the anodic oxide coating easily becomes uneven in color and that a uniform white-based anodic oxide coating color is not easy to obtain. Therefore, an aluminum alloy sheet which is based on the 1000-series and is improved in uniformity of the roughened surface and uniformity of the color of the anodic oxide coating and a method of production of the same have been developed.
PLT 2 describes an aluminum alloy sheet which contains Fe: 0.2 to 0.6 wt %, Si: 0.03 to 0.20 wt %, and Ti: 0.005 to 0.05 wt % and has a balance of Al and unavoidable impurities and a method of production of the same. According to this, by suitably adjusting the contents of the chemical components Fe, Si, and Ti of the materials and in particular making the Fe/Si ratio a suitable range, the formation of metastable phases is suppressed, the intermetallic compounds become mainly stable phases, and the uniformity of pits formed by roughening treatment and the uniformity of color of the anodic oxide coating are remarkably improved.
An alloy sheet based on the 1000-series is excellent in uniformity of etch pits formed during alkali etching by caustic soda and in uniformity of color of the anodic oxide coating and is excellent in shapeability, but has the problem of a low strength. Therefore, while a white-based anodic oxide coating color is being sought, high strength characteristics are also expected to be demanded, so application of an aluminum material based on the 1000-series would be a problem.
Therefore, an Al—Mg-based 5000-series alloy sheet for anodic oxide coating excellent in high strength characteristics has been developed.
PLT 3 proposes an aluminum alloy sheet which contains, by mass %, Mg: 2.0 to 3.0%, Cr: 0.15 to 0.25%, Ti: 0.005 to 0.20%, or Ti: 0.005 to 0.20% and B: 0.0005 to 0.05%, has a balance of Al and unavoidable impurities, restricts as impurities the Si to 0.15% or less, Fe to 0.4% or less, and Mn to 0.06% or less, and, when making the content of the Cr TCR % and the solid solution amount of Cr SCR %, PCR=TCR−SCR≤0.065%.
According to this, by restricting the Cr-containing intermetallic compounds Cr to a predetermined level or less, it is said that an Al—Mg-based alloy sheet which contains Cr enables the production of an aluminum alloy sheet which forms a light green white color kept down in yellowishness even if forming an anodic oxide coating by a sulfuric acid bath.